Neurophilosophy

IF a rapid series of taps are applied first to your wrist and then to your elbow, you will experience a perceptual illusion, in which phantom sensations are felt along the skin connecting the two points that were actually touched. This feels as if a tiny rabbit is hopping along your skin from the wrist to the elbow, and is therefore referred to as the “cutaneous rabbit”. The illusion indicates that our perceptions of sensory inputs do not enter conscious awareness until after the integration of events occuring within a certain time window, and that the sensory events taking place at a certain point can be influenced by future events.

A group of Japanese researchers now shows that this illusion is not just confined to the body. In a new study published today in the Journal of Neuroscience, they report that the cutaneous rabbit can easily be induced to “hop out” of the body, so that the illusory sensations are perceived to originate not from the body itself, but from external objects that interact with it.

The cutaneous rabbit illusion was first described in 1972 by psychologists Frank Geldard and Carl Sherrick, who were then at Princeton University’s Cutaneous Communications Laboratory. Geldard and Sherrick discovered the phenomenon by accident, while designing experiments to investigate the perception mechanical stimuli applied to the forearm. The underlying mechanisms, however, remained unknown for years afterwards. Some researchers attributed the illusion to activity in the primary somatosensory cortex, the first stop in the brain for tactile sensations entering from the body, while others argued that cognitive processes such as selective attention are involved. This was settled in 2006, with the publication of a functional neuroimaging study by researchers from UCL, which showed that the phantom tactile sensations perceived during the illusion are associated with activity in the corresponding regions of the somatosensory cortex.

In the new study, Makoto Miyazaki of the Kochi University of Technology and his colleagues show that the cutaneous rabbit is not confined to the body, but can “hop out”, so that the illusory sensations are perceived to be emanating from an object. They recruited 8 participants and sat them at a table in front of a device consisting of a number of piezoelectric contactors attached to a flat 10cm-long aluminium rod. The participants wore earphones which emitted beeps; each time they heard a beep, they were required to place the tips of their index fingers onto the underside of the aluminium rod and close their eyes. The device then applied trains of mechanical pulses to one index finger and then the other, in quick succession. A second experiment, in which the pulses were delivered by small metal plates instead of a stick, was also performed.

At the end of each trial, the participants used a set of pointers to indicate where the tactile sensations they “felt” had come from. In the first experiment, but not the second, all the participants experienced the cutaneous rabbit, and reported that they felt sensations from two points on the aluminium stick as well as from their fingertips. The cutaneous rabbit had hopped out of the body and onto the aluminium stick.

How can this illusion occur, given that the stick lacks a corresponding receptive field in the somatosensory cortex? It can be explained in terms of the body schema (or body image), a representation of the physical body encoded by the brain. It has long been known that the body schema can be temporarily extended – the brain temporarily incorporates external objects such as clothes and tools into the schema, treating them as if they were a part of the body. Thus, the cutaneous rabbit illusion occurs because the brain regards the aluminium stick as an extension of the fingers, and incorporates it into the body schema. As a result, the stimuli applied to the fingertips are mislocalized, and perceived to originate from the stick and not from the fingers.

Related:

Miyazaki, M., et al. (2010). The “Cutaneous Rabbit” Hopping out of the Body. J. Neurosci. 30: 1856-1860. DOI: 10.1523/jneurosci.3887-09.2010.[Full text]

Blankenburg, F., et al (2006). The Cutaneous Rabbit Illusion Affects Human Primary Sensory Cortex Somatotopically. PLoS Biol 4 (3): e69. DOI: 10.1371/journal.pbio.0040069. [Full text]

Comments

#1 TC
February 4, 2010

Is this how my phantom limb pains happen?
#2 steve
February 4, 2010

At the risk of being gullible, (yes, April Fool’s day approaches) I would really like to know how the first tapping exercise is supposed to be conducted.

Yes, I am that guy over there tapping on his wrist and elbows with a confused look on his face.

Any help?
#3 Simon
February 4, 2010

Nope, I’m manically tapping too and nothing. Perhaps my cutaneous bunnies have died?
#4 wazza
February 4, 2010

A lot of these sorts of things can only be triggered by someone else, in the same way that you can’t tickle yourself. Get a friend to try it.
#5 Janne
February 4, 2010

You can’t tap yourself, or have taps directly under your control I believe. Your brain knows it is the cause of the sensation and discounts them.
#6 Nigel
February 4, 2010

Some researchers attributed the illusion to activity in the primary somatosensory cortex, the first stop in the brain for tactile sensations entering from the body, while others argued that cognitive processes such as selective attention are involved. This was settled in 2006, with the publication of a functional neuroimaging study by researchers from UCL, which showed that the phantom tactile sensations perceived during the illusion are associated with activity in the corresponding regions of the somatosensory cortex.

Are you implying that the fact that primary somatosensory cortex is activated rules out a role for attention in this phenomenon? (The paper you link to does not seem to say so.) I do not know enough about somatosensory cortex to be sure, but primary visual cortex can certainly be activated, top-down, by attention. See, for example:

Munneke, J., Heslenfeld, D.J., & Theeuwes, J. (2008). Directing Attention to a Location in Space Results in Retinotopic Activation in Primary Visual Cortex. Brain Research, 1222, 184-191.

Bressler, S.L., Tang, W., Sylvester, C.M., Shulman, G.L., & Corbetta, M. (2008). Top-Down Control of Human Visual Cortex by Frontal and Parietal Cortex in Anticipatory Visual Spatial Attention. Journal of Neuroscience, 28 #40, 10056-10061.
#7 botogol
February 5, 2010

I think you can demonstrate this effect to some extent simply using a screwdriver, and focusing on the sensation of the tip and the screw.
#8 Mo
February 5, 2010

@TC: See this post for a hypothesis about phantom limbs.

@Steve and Simon: Don’t try this at home – not because it’s dangerous, but because it probably won’t work. As Wazza and Janne say, the tapping has to be precisely controlled.

@Nigel: I’m not saying that somatosensory activity and selective attention, etc. are mutually exclusive.
#9 Staci
February 5, 2010

I am an above knee amptuee and have severe phantom limb pain . After I already endured 68 operations, 23 broken bones, over 1400 days in the hospital then lost my leg and had 2 revisions. Living with one leg is a piece of cake compared to the PLKP which is crushing, burning, excruciatingly unbearably effin insanity. It feels like a vice is enclosing tighter and tighter. I have tried 40 different meds and the side effects aren’t that easy to roll with. I just went on lyrica mid Dec and with my one long acting opiate am doing much better. i plan to try a spinal cord stimulator.
Ok now what I wanted to say is if my little yorkie licked my right palm in certain areas I felt all these strange sensations in my phantom foot where my pain is. Now if I tickle my palm or even my husband does nothing happens. I think V.S. Ramachandran would be interested in this and boy would I love to meet this genius. I have honestly researched chronic pain over 1000 hours because when you are in pain that’s at the degree of almost going out of your mind, you become consumed with finding a solution and there isn’t much for phantom pain. I have tried the mirror therapy and the reason I think it failed is because my brain hasn’t seen a normal looking leg on the right side since i was 13 in 1981. I am open to suggestions and will drive or fly anywhere. My email address is
stacilyn68@yahoo.com
Thanks, Staci
#10 Eric Thomson
February 14, 2010

I’m not sure we need to invoke the body schema idea here. If I tap a metal bar that you are touching, you will feel it obviously, because it activates your sensory transducers where you are touching it, in a unique way for different locations on the bar.

I have a related technical question about the study. Did they just ask the participants what it subjectively felt like, or was there a comparison control where the metal bar was actually touched on some trials, and subjects had to say whether the metal bar was actually touched versus their fingers only touched?

For the standard sub-c rabbit running up the arm, researchers now have an improved version of the task where they either tap up the arm, or just two locations on the arm, and subjects often can’t tell the difference as revealed by the psychophysics.
#11 @murmur55
February 16, 2010

This illusion is related to the “phenomenal self model” of Metzinger, I would guess. He suggests that an avatar which is actively working at all times can be manipulated so that out of body experiences can occur. His example to get a sense of the avatar: think of how you think you “see” with your eyes, when actually you are using your occipital cortex to render the images in the environment. The avatar takes a shortcut to make it seem that the eyes are “doing” the seeing as they move about hunting for images.
#12 Mo
February 18, 2010

@Eric: They only used the participants’ subjective reports. The control was the condition in which they placed their fingertips separately on metal plates instead of the stick.